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Wing mounted jet engines face the free stream at Mach number 1 or above, and they work perfectly for Mach numbers below 1 as well, even though the free stream characteristics change a lot after Mach 1.

  1. How do the engines manage to work despite the fact that after Mach 1, the Shock waves would try and disrupt the normal airflow?
  2. If the engines are optimized to work at above Mach 1, does this cause any trouble for them to work under subsonic conditions?

Don't confuse with "Why cant jet engines work with supersonic flow?"

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    $\begingroup$ you cannot simply add a line like that: Don't confuse with "Why cant jet engines work with supersonic flow?" why? what makes this question different? $\endgroup$
    – Federico
    Jun 24, 2015 at 20:41
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    $\begingroup$ Which aircraft which has wing-mounted engines and is capable of supersonic flight do you have in mind? Because I can't think of any (assuming you mean hung in pods below and ahead of wings and not stuck to underside like Concorde or B-1B, which look very different)! $\endgroup$
    – Jan Hudec
    Jun 24, 2015 at 20:44
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    $\begingroup$ The answers to the linked question do actually answer this question. $\endgroup$
    – reirab
    Jun 24, 2015 at 20:47

1 Answer 1

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Flow conditions are not so different, only the reaction of the flow to an obstacle is. By shaping the intake appropriately, supersonic flow is handled easily, and shocks will be helpful in decelerating and compressing the flow.

However, supersonic flow demands sharp intake lips which are hit by the flow at the right angle, otherwise drag rises quickly to intolerable levels. These sharp lips will cause flow separation in subsonic flow when hit by the flow at an angle, so aerodynamic tricks like bypass doors need to be added to the intake to keep flow angles at the intake lips in check.

Note that the intake will ensure a flow speed around Mach 0.5 at the compressor face of the engine, regardless of flight speed. Therefore, the engine will not need to be optimized for supersonic flight. However, to keep its frontal area low and the nozzle speed high, its bypass ratio should be low. The only troubles this causes are increased engine fuel consumption and noise.

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  • $\begingroup$ " ... its bypass ratio should be low". A-ha. Watching an old video about B-58 Hustler development I think, it mentioned offhandedly that some design consideration allowed to the bypass ratio to be kept low. I thought that odd, now I see. $\endgroup$
    – radarbob
    Jun 24, 2015 at 15:09

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